The present invention relates to a process for reducing the microbial count in microbially-contaminated aqueous multi-phase compositions that contain synthetic resin, such as polymer dispersions and formulations that comprise polymeric binders, by dielectric heating.
Aqueous multi-phase systems that contain a synthetic resin, for example, polimer dispersions and dispersion-bonded systems for the adhesives or paints sector, are threatened by microbial attack and microbial decomposition. Temperatures of about room temperature and pH values in the neutral to slightly alkaline range represent good growth conditions for bacteria, yeasts and fungi. These microorganisms are everywhere in the environment. Various microorganisms have been detected in dispersions of plastics (K. H. Wallhauser, W. Fink, Die Konservierung von wa.beta.rigen Kunststoff-Dispersionen, [PRESERVATION OF AQUEOUS PLASITCS DISPERSIONS], Farbe und Lack 91, 277 (1985)); bacteria: Pseudomonas, Enterobacteriaceae, Bacillus species fungi: Aspergillus, Penicillium, Cephalosporium, Fusarium; yeasts: Saccharomyces. Uninhibited growth of such microorganisms leads to clearly perceptible external changes, for example, the occurrence of an unpleasant smell or visible mold growth, as well as to a change in important applications properties, for example, the viscosity or the pH.
The desired value for the microbial count is not more than 1000 microbes per m1, a value which complies with the guidelines of the Drinking Water Act of 1986 (BGB1. I No. 22, 760, 1986) for water at 18-22.degree. C. from collection and storage tanks.
Various chemical and physical methods are described for preventing attack or for reducing the risk of an attack by microorganisms, for example, in Wallhauser, Praxis der Sterilisation, Desinfektion-Konservierung [Practice of sterilization and disinfection/preservation], 4th Edition, Georg Thieme Verlag, Stuttgart, 1988. The method most frequently used in practice for preservation or sterilization entails the addition of substances that have a microbicidal action, such as bactericides and fungicides. The chemical preservatives used are subject to strict approval proceedings, and therefore only a limited number are available. Because of toxic or allergenic properties they may be employed only in small amounts, but can nevertheless cause health disorders in sensitive persons in isolated cases. Particularly active preservatives, for example, formaldehyde or products which split off formaldehyde, may not be used at all within wide fields of use for a formulation according to BGA [Federal Board of Health] guidelines. Another problem in the use of chemical preservatives is the increasing resistance of various microbes.
Aqueous multi-phase compositions that contain synthetic resin, such as polymer dispersions, dispersion adhesives, emulsion paints for internal and external use, primers and plasters based on dispersions, concrete auxiliaries, paper-coating compositions, leather finishes, cheese-covering compositions and dispersions of polycondensation products, which are prepared in situ or as a secondary dispersion, have a complex colloidchemical structure which protects them from phase separation on the basis of electrostatic, steric or electrosteric stabilization mechanisms. To obtain the desired technical properties, this system must persist in particular in respect of its material composition and its state of division during preparation, storage and transportation up to application. This is generally no longer the case after a massive multiplication of microorganisms. A number of physical and chemical treatment methods therefore already have been tried, to avoid the use of microbicides or to limit them in terms of amount. Sterilization by means of known physical methods, for example, by heat treatment or by treatment with .gamma.-rays, produces a change in this complex colloid-chemical structure. Heat treatment of a polymer dispersion by means of indirect heating leads to adequate microbe reduction only at high temperatures over long residence times (121.degree. C., 3 min). However, after only a short time, a heavy deposit forms on the heat exchanger walls, a coagulate forms or the particle size distribution changes due to a disturbance in the colloid-chemical composition. The formation of a deposit then produces further overheating as a result of an increased heat transfer resistance, and therefore to increased deposits. These deposits comprise, for example, coagulated polymer particles which can no longer be redispersed in the case of heat treatment of a dispersion, and coagulated polymer particles which have enclosed pigments and/or fillers in the case of heat treatment of a paint. The use of .gamma.-rays, for example, from a .sup.60 Co source, in the presence of oxygen and water leads to formation of hydrogen peroxide and of hydroxyl radicals, which trigger undesirable side reactions and cause crosslinking of the polymer. These physical stabilization methods are therefore unsuitable for an industrially feasible sterilization of aqueous multi-phase compositions that contain synthetic resin.
The use of microwave radiation for heating is known for sterilization in the foodstuffs sector and for pharmaceutical products (K. H. Wallhauser, PRAXIS DER STERILISATION [PRACTICE OF STERILIZATION] (4th Edition), Desinfektion-Konservierung [Disinfection-Preservation], georg Thieme Verlag, Stuttgart, 1988, p. 250). Koch also describes the use of microwave technology for sterilizing foodstuffs (K. Koch, UBER DEN EINSATZ DER MIKROWELLENTECHNIK ZUM HALTBARMACBEN VON LEBENSMITTELN [THE USE OF MICROWAVE TECHNOLOGY FOR PRESERVING FOODSTUFFS], Lebensmitteltechnik, 1-2, pages 39-43, 1989).
In the foodstuffs sector, microwave radiation is used exclusively to heat the products to be sterilized to a certain temperature. The exposure time needed for inactivation/destruction of the microorganisms must be ensured, for example, in a subsequent heat retention tunnel. The temperature of at least 121.degree. C. mentioned in the literature which is necessary for sterilizing corresponds precisely to the temperature which is necessary for sterilizing by means of other heat processes. Based on this fact, this method should be unsuitable for the complex multi-phase compositions that contain synthetic resin, since the high temperatures required trigger the changes already described, in particular coagulation of the complex composition.
The use of microwave beams for the preparation of polymer dispersions is already known from U.S. Pat. No. 3 432 413. The reduction of the residual monomer content in vinyl halide dispersions by means of microwave radiation is described in U.S. Pat. No. 4 117 220 and DE-A 26 12 572.